Method for non-flutter transmission of digital data in a free-space optical transmission system

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of International Application No. PCT/EP2009/052220 filed Feb. 25, 2009, which designates the United States of America, and claims priority to DE Application No. 10 2008 011 823.0 filed Feb. 29, 2008. The contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The field of the invention is communications and relates to a method for transmitting digital data in a free-space optical transmission system.

BACKGROUND

Against the background of a continuously growing volume of data, optical transmission systems are being increasingly used, as they provide very high data rates compared to the bandwidth of electric cables. In optical transmission systems, light waves modulated with the data to be transmitted are generated, coupled into an optical waveguide generally consisting of glass or polymers, and transmitted to the receiver. In practice, wireline optical transmission systems often lack a desirable degree of flexibility.

In recent years, wireless optical transmission systems commonly known as “free-space optical transmission systems” are being increasingly considered for practical applications. In free-space optical transmission systems, the modulated light waves are not coupled into an optical waveguide, but launched into free space and received by a photodetector.

For optical transmission of digital data (binary data), the data is converted in two steps, a serial code sequence (coded sequence of binary data) first being produced by means of an encoding technique and then modulated onto the light waves using a modulation scheme (amplitude, frequency or phase modulation).

Encoding (“line coding”) of the digital data is performed in practice in two ways: either the individual binary values or else the transitions of binary values are encoded as signal levels. In the first case, a so-called NRZ technique (NRZ=non-return-to-zero-level), for example, is used in which a logical “0” is encoded with a low signal level and a logical one “1” with a high signal level. In the second case, so-called Manchester coding is used, for example, whereby each bit period is split into two halves, a logical “1” being represented by a low signal level at the start and a high signal level at the end, and a logical zero “0” in a correspondingly vice versa manner.

If NRZ-encoded data is converted into an amplitude-modulated optical signal, a logical “0” is transmitted with a low signal level (light of little or no intensity) and a logical “1” with a high signal level (light of high intensity). If longer sections (bit blocks) each with the same logical value occur in an NRZ-encoded bit sequence, e.g. “1111110000000”, which contains two longer bit blocks of the logical values “1” and “0”, the light intensity of the optical signal remains unchanged during these bit blocks. In the case of optical transmission in the visible wavelength range, this may be perceived by the viewer as flutter or flicker if the light wave modulation frequency determined by the bit sequence is less than approximately 70 hertz. This can cause the viewer discomfort and even trigger epileptic fits, as medical studies have shown.

SUMMARY

According to various embodiments, a method for transmitting digital data in a free-space optical transmission system can be provided which enables an optical signal to be transmitted without flutter or flicker in the visible wavelength range.

According to an embodiment, in a method for transmitting digital data in a free-space optical transmission system, amplitude-modulated light waves are transmitted over a free-space optical link between a transmitter and a receiver using a binary code sequence generated from the digital data, wherein the binary code sequence is generated such that a modulation frequency of the light waves which is determined by the binary code sequence is at least 70 hertz.

According to a further embodiment, the binary code sequence can be generated such that a modulation frequency of the light waves which is determined by the binary code sequence is at least 100 hertz. According to a further embodiment, light waves with a visible wavelength can be transmitted. According to a further embodiment, the method may comprise the following steps: —generating a first binary code sequence from the digital data in an encoder by means of an encoding technique; —converting the first binary code sequence into a second binary code sequence in a code sequence converter by means of a code conversion scheme such that a light wave modulation frequency determined by the second binary code sequence is at least 70 hertz, wherein, in the code conversion scheme, first binary data blocks of selectable block length are replaced by selectable second binary data blocks assigned to the first binary data blocks on a one-to-one basis; —modulating the intensity of the light waves with the second code sequence in a modulatable light source of the transmitter; —receiving and demodulating the light waves in the receiver to recover the second code sequence; —converting the second code sequence back into the first code sequence in a code sequence back-converter by means of the inverse code conversion scheme, wherein the second binary data blocks are replaced by the first binary data blocks; and —generating the digital data from the first binary code sequence using the inverse encoding technique. According to a further embodiment, the binary second code sequence can be selected such that a light wave modulation frequency determined by the binary second code sequence is at least 100 hertz. According to a further embodiment, the first binary code sequence can be generated from the digital data by means of a non-return-to-zero encoding technique. According to a further embodiment, the second binary code sequence can be generated by 4B/5B code conversion. According to a further embodiment, the second binary code sequence can be generated by 8B/10B code conversion. According to a further embodiment, the binary code sequence can be generated from the digital data by means of a return-to-zero encoding technique. According to a further embodiment, the binary code sequence can be generated from the digital data by means of a pulse position modulation technique.

According to another embodiment, in a method for transmitting digital data in a free-space optical transmission system, wherein amplitude-modulated visible light waves are transmitted over a free-space optical link between a transmitter and a receiver using a binary code sequence generated from the digital data, the method may comprise the following steps: —generating the binary code sequence from the digital data to be transmitted; —defining a guard interval with a selectable duration and subdividing the guard interval into at least a first and second time section; —defining a setpoint quantity of light to be transmitted during a same guard interval; —subdividing the binary code sequence into binary data blocks with a selectable block length; —modulating the intensity of the light waves with the binary code sequence and transmitting the modulated light waves binary data block by binary data block over the free-space link during the first time sections of the guard intervals; —determining for each guard interval the quantity of light to be transmitted or which is transmitted during the first time section, possibly summed for a plurality of binary data blocks, and calculating for each guard interval a quantity of light difference between the setpoint quantity of light and the quantity of light to be transmitted or which is transmitted during the first time section; —transmitting unmodulated light waves during the second time sections of each guard interval, the light intensities of the light waves being selected such that the light quantities transmitted correspond in each case to the quantity of light differences calculated.

According to a further embodiment of the above method, the guard interval may be no more than 14 milliseconds long. According to a further embodiment of the above method, the guard interval may be no more than 10 milliseconds long. According to a further embodiment of the above method, only light waves encoded with a single binary data block may be transmitted during the first time sections of the guard intervals in each case. According to a further embodiment, the light quantities transmitted during the first time sections of the guard intervals can be determined computationally. According to a further embodiment, the light quantities transmitted during the first time sections of the guard intervals can be determined by means of a photodetector.

According to yet another embodiment, a device for transmitting digital data in a free-space optical transmission system, may comprises means which are implemented such that a method as described above can be carried out.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in greater detail using an exemplary embodiment and with reference to the accompanying drawings in which:

<figref idrefs="DRAWINGS">FIG. 1</figref> schematically illustrates a free-space optical transmission system;

<figref idrefs="DRAWINGS">FIGS. 2</figref><i>a</i>-<b>2</b><i>b </i>shows NRZ encoding and 4B/5B conversion of a typical binary data sequence according to a first exemplary embodiment.

<figref idrefs="DRAWINGS">FIG. 3</figref> shows a method for transmitting digital data in a free-space optical transmission system using a sequence of guard intervals each having a first time section and a second time section, wherein the digital data is transmitted as modulated light in the first time sections and non-modulated light is transmitted in the second time sections.

DETAILED DESCRIPTION

The various embodiments show, in generic terms, a method for transmitting digital data in a free-space optical transmission system in which in particular visible light waves are amplitude- or intensity-modulated with a binary code sequence generated from the digital data, and then transmitted over a free-space optical link between a transmitter and a receiver. A binary code sequence is defined as a sequence of binary values encoded by means of an encoding scheme.

According to a first aspect, the method according to various embodiments is essentially characterized in that the binary code sequence is generated such that a light wave modulation frequency determined by the binary code sequence, i.e. a rate of change of the signal levels (light intensities) of the light waves, is at least 70 hertz, preferably at least 100 hertz. The figure in hertz corresponds to signal level changes per second. The bit period, i.e. the time available for the optical transmission of a binary value, must be suitably selected to achieve the desired modulation frequency.

By means of the method according to a first aspect, visible optical signals can advantageously be transmitted flutter- or flicker-free over a free-space optical transmission system.

An further embodiment of the method comprises the following successive steps:

A first binary code sequence is first generated from the digital data to be transmitted in a transmitter-end encoder using a selectable encoding technique. The first binary code sequence is generated from the digital data using, for example, a non-return-to-zero encoding scheme.

The first binary code sequence is then converted into a second binary code sequence in a transmitter-end code sequence converter using a code conversion technique such that a light wave modulation frequency determined by the second binary code sequence is at least 70 hertz, preferably at least 100 hertz. In the code conversion method, first binary data blocks (bit blocks) of a selectable (identical) block length are replaced by selectable second binary data blocks which are assigned to the first binary data blocks on a one-to-one basis.

For example, 4B/5B code conversion (4B/5B=4 Binary/5 Binary) or 8B/10B code conversion (8B/10B=8 Binary/10 Binary) is carried out as the code conversion scheme. In the case of 4B/5B code conversion, a block of 4 binary values is converted into a block of 5 binary values such that no more than two consecutive logical zeros occur in each 5-bit block. In this process, 4-bit blocks are in each case assigned 5-bit blocks defined according to this requirement by means of a truth table. In the case of 8B/10B code conversion, a block of 8 binary values is converted into a block of 10 binary values such that a logical “0” occurs five times and a logical “1” occurs five times in each 10-bit block, at least three state changes being involved. In this process the 10-bit blocks are each assigned to 8-bit blocks by means of a truth table.

Next, the light wave intensity is modulated with the second code sequence in a modulatable light source of the transmitter. The light waves launched at the transmitter end are received and demodulated in the receiver to recover the second code sequence.

The second code sequence is then converted back into the first code sequence in a code sequence back-converter using the inverse code conversion scheme, the second binary data blocks being replaced by the first binary data blocks.

The digital data is then generated from the first binary code sequence using an inverse encoding technique.

This embodiment of the method is characterized by particularly simple technical implementation.

In another embodiment of the method, the binary code sequence is generated from the digital data by means of a return-to-zero encoding technique known per se to those skilled in the art. In the return-to-zero encoding scheme, a logical “0” is encoded with a low signal level and a logical one “1” is encoded with a high signal level, changeover to the low signal level taking place if a logical “1” is transmitted before the end of the pulse. The bit period is appropriately adjusted to achieve the required modulation frequency.

This embodiment of the method is characterized by another particularly simple technical implementation.

In another embodiment of the method, the binary code sequence can be generated from the digital data by means of a pulse position modulation technique known per se to those skilled in the art.

This embodiment of the method is characterized by another particularly simple technical implementation.

According to a second aspect, a generic method may comprise the following non-successive steps:

The binary code sequence is generated by means of a selectable encoding technique from the digital data to be transmitted.

The first binary code sequence is generated, for example, from the digital data using a non-return-to-zero encoding scheme.

A time period with a selectable duration, referred to here and in the following as a “guard interval”, is specified which is subdivided into at least two time sections, a first time section and a second time section, with selectable durations.

The duration of an individual guard interval is preferably no more than 14 milliseconds, or even more preferably no more than 10 milliseconds. A setpoint quantity of light to be transmitted during a same guard interval is defined for the guard interval.

The binary code sequence generated is subdivided into sequential binary data blocks with a selectable block length that is preferably the same in each case.

The intensity or amplitude of the light waves is modulated with the binary code sequence generated and the modulated light waves are transmitted binary data block by binary data block over the free-space link during the first time sections of the guard intervals. In this case one or more binary data blocks (or rather the light waves modulated therewith) can be transmitted during a same first time section.

The quantity of light to be transmitted or transmitted during the first time section is determined for each binary data block and, in the event that a plurality of binary data blocks are transmitted during a same first time section, the light quantities to be transmitted or which are transmitted during a same first time section are summed for each guard interval. In the event that only a single binary data block (or rather the light waves encoded with said binary data block) is transmitted during a same first time section, the summation can be omitted. A quantity of light difference between the specified setpoint quantity of light and the possibly summed quantity of light to be transmitted or which is transmitted during the first time section is then calculated for each guard interval. The light quantities to be transmitted or which are transmitted during the first time sections of the guard intervals for each binary data block can be determined computationally at the transmitter end or by measurement by a photodetector at the receiver end.

During the second time sections of each guard interval, unmodulated light waves are transmitted, the light intensities of said light waves being selected such that the quantities of light transmitted in this case each correspond to the quantity of light differences calculated for a guard interval. It must be ensured that the duration of the second time section is not so short that the light waves cannot reach the receiver, i.e. the duration of the second time section must be longer than an inverse modulation bandwidth of the light source. In addition, the optical power necessary to produce the quantity of light difference must be selected such that a permissible maximum power for a light source generating the light waves is not exceeded.

The method according to a second aspect advantageously enables visible optical signals to be transmitted by a free-space optical transmission system without flutter or flicker that is perceptible to a viewer.

In a technically simple-to-implement embodiment of the method according to a second aspect, only a single binary data block (or rather the light waves encoded with that binary data block) is ever transmitted during the first time sections of the guard intervals in each case. Nevertheless, it may be preferable for a plurality of binary data blocks (or rather the light waves encoded with those binary data blocks) to be transmitted during the first time sections of the guard intervals in each case. In the event that a calculated quantity of light difference is zero, it is not necessary for unmodulated light waves to be transmitted during the second time sections of the guard intervals.

According to a another embodiment, a device, e.g. a free-space optical transmission system, for transmitting digital data in a free-space optical transmission system, may comprise means such as a transmitter with an amplitude-modulatable light source and a receiver with a photodetector which are designed such that a method as described above can be carried out.

A first exemplary embodiment according to a first aspect will now be described:

<figref idrefs="DRAWINGS">FIG. 1</figref> schematically illustrates a free-space optical transmission system denoted as a whole by the reference numeral <b>100</b>. The free-space optical transmission system <b>100</b> comprises a transmitter <b>101</b> with an amplitude-modulatable light source (e.g. a laser diode) <b>103</b> and a receiver <b>107</b> with a photodetector (e.g. a photodiode) <b>108</b>.

The light source <b>103</b> is preceded by a combined encoder/converter <b>102</b> to which digital data (binary data) <b>104</b> is fed via a signal input (not shown).

In a first step, the binary data <b>104</b> undergoes NRZ encoding in the encoder/converter <b>102</b>, resulting in the generation of a first serial code sequence (A) in the form of a serial sequence of encoded binary data to which, in a second step, 4B/5B conversion is then applied to produce a second serial code sequence (B) <b>105</b>.

The second serial code sequence <b>105</b> is modulated onto a light beam <b>106</b> with a wavelength of e.g. 633 nm generated in the amplitude-modulatable light source, which light beam is launched into free space in the direction of the receiver <b>107</b>. In the photodetector <b>108</b> of the receiver <b>107</b>, the light beam is received and demodulated to recover the second serial code sequence B <b>105</b>.

The photodetector <b>108</b> is followed by a combined decoder/back-converter <b>109</b> to which the recovered second code sequence (B) <b>105</b> is fed via a signal input (not shown). In the decoder/back-converter <b>109</b>, the second code sequence (B) <b>105</b> is converted back into the first code sequence (A) by performing inverse 4B/5B conversion. The binary data <b>104</b> to be transmitted is recovered from the first code sequence (A) by applying the inverse NRZ encoding technique, said data being provided at a signal output (not shown) of the decoder/back-converter <b>109</b>.

In <figref idrefs="DRAWINGS">FIG. 2</figref>, the encoding/conversion of the binary data <b>104</b> performed in the encoder/converter <b>102</b> is explained in more detail with reference to a typical bit string “000110000010”. The upper diagram shows the NRZ encoding of this bit string, each logical “0” being encoded with a low signal level and each logical “1” with a high signal level to produce the binary code sequence (A). The result of this is that the binary code sequence (A) contains longer sections with the same signal level—here a sequence of five low signal levels, for example.

The NRZ-encoded binary code sequence (A) then undergoes 4B/5B conversion to produce the second serial code sequence (B). For the application of 4B/5B conversion, the binary code sequence (A) is subdivided sequentially into 4-bit blocks (binary words) each with 4 following binary values. The 4-bit blocks contained are replaced by 5-bit blocks which are allocated to the 4-bit blocks according to the following one-to-one truth table.

<tables id="TABLE-US-00001" num="00001"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="3"><colspec colname="offset" colwidth="49pt" align="left" /><colspec colname="1" colwidth="35pt" align="center" /><colspec colname="2" colwidth="133pt" align="center" /><thead><row><entry /><entry namest="offset" nameend="2" align="center" rowsep="1" /></row><row><entry /><entry>4-bit block</entry><entry>5-bit block</entry></row><row><entry /><entry namest="offset" nameend="2" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /><entry>0000</entry><entry>11110</entry></row><row><entry /><entry>0001</entry><entry>01001</entry></row><row><entry /><entry>0010</entry><entry>10100</entry></row><row><entry /><entry>0011</entry><entry>10101</entry></row><row><entry /><entry>0100</entry><entry>01010</entry></row><row><entry /><entry>0101</entry><entry>01011</entry></row><row><entry /><entry>0110</entry><entry>01110</entry></row><row><entry /><entry>0111</entry><entry>01111</entry></row><row><entry /><entry>1000</entry><entry>10010</entry></row><row><entry /><entry>1001</entry><entry>10011</entry></row><row><entry /><entry>1010</entry><entry>10110</entry></row><row><entry /><entry>1011</entry><entry>10111</entry></row><row><entry /><entry>1100</entry><entry>11010</entry></row><row><entry /><entry>1101</entry><entry>11011</entry></row><row><entry /><entry>1110</entry><entry>11100</entry></row><row><entry /><entry>1111</entry><entry>11101</entry></row><row><entry /><entry namest="offset" nameend="2" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

4B/5B conversion of binary code sequence (A) into binary code sequence (B) for the typical bit string is shown in the lower part of <figref idrefs="DRAWINGS">FIG. 2</figref>. It can be seen that longer sections of the same signal level are no longer present in the binary code sequence (B).

With a correspondingly selected bit period, this means that the modulation frequency for the intensity modulation of the light beam <b>106</b> is greater 100 hertz, so that more than 100 intensity changes occur per second and the light <b>106</b> beam is not perceived to flutter or flicker by the viewer.

A second exemplary embodiment according to a second aspect will now be described with reference to <figref idrefs="DRAWINGS">FIG. 3</figref>:

Let it be assumed that a bit string “000110000010” generated from digital data (Step <b>301</b>) is to be transmitted over a free-space optical link of a free-space optical transmission system as shown in <figref idrefs="DRAWINGS">FIG. 1</figref>. For this purpose the bit string is first NRZ-encoded to produce a binary code sequence, as shown in <figref idrefs="DRAWINGS">FIG. 2</figref>, upper section.

The binary code sequence is then subdivided into three four-bit blocks to produce the bit blocks “0001”, “1000”, “0010”. (Step <b>302</b>)

Each bit block is to be transmitted during a separate guard interval for which a duration of 10 milliseconds is specified. (Step <b>303</b>) It is also specified that each guard interval is subdivided into two time sections of five milliseconds each, a bit block being transmitted during the first time section of a guard interval in each case. (Step <b>304</b>) For the guard interval, a setpoint quantity of light (unit: joule or lumen-seconds) to be transmitted during the guard interval is specified. (Step <b>305</b>) For example, <b>1</b> lumen-second is specified as the setpoint quantity of light for the guard interval here.

The quantity of light to be transmitted for each bit block is determined computationally at the transmitter end, which can be performed in a simple manner on the basis of the light intensities and bit periods used. (Steps <b>306</b> and <b>307</b>) For example, the quantity of light transmitted for the three bit blocks is 0.4 lumen-seconds in each case.

For optical transmission of the binary code sequence, the first bit block is first modulated and transmitted with a total quantity of light of 0.4 lumen-seconds during the first time section of the first guard interval. (Step <b>308</b>) This is followed by the transmission of unmodulated light waves with a total of 0.6 lumen-seconds during the second time section of the first guard interval. (Step <b>309</b>) The second bit block is transmitted in a corresponding manner, wherein amplitude-modulated light waves are transmitted during the first time section of the second guard interval with the second bit block, followed by transmission of unmodulated light waves during the second time section of the second guard interval with a light intensity resulting in a transmitted quantity of light of 0.6 lumen-seconds. Similarly, the third bit block is transmitted in a corresponding manner, wherein amplitude-modulated light waves are transmitted during the first time section of the third guard interval with the third bit block, followed by transmission of unmodulated light waves during the second time section of the third guard interval with a light intensity resulting in a transmitted quantity of light of 0.6 lumen-seconds.

<tables id="TABLE-US-00002" num="00002"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="1"><colspec colname="1" colwidth="217pt" align="center" /><thead><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row><row><entry>List of reference characters</entry></row><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="2"><colspec colname="1" colwidth="91pt" align="center" /><colspec colname="2" colwidth="126pt" align="left" /><tbody valign="top"><row><entry>100</entry><entry>free-space transmission system</entry></row><row><entry>101</entry><entry>transmitter</entry></row><row><entry>102</entry><entry>encoder/converter</entry></row><row><entry>103</entry><entry>light source</entry></row><row><entry>104</entry><entry>binary data</entry></row><row><entry>105</entry><entry>code sequence</entry></row><row><entry>106</entry><entry>light beam</entry></row><row><entry>107</entry><entry>receiver</entry></row><row><entry>108</entry><entry>photodetector</entry></row><row><entry>109</entry><entry>decoder/back-converter</entry></row><row><entry namest="1" nameend="2" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>